In mobile devices in recent years, such as cellular phones, a transparent or a translucent liquid-crystal display is mainly adopted, and power for a backlight of the liquid-crystal display part accounts for the majority of power of the entire module. And therefore, innovation for reducing the power for a backlight is required.
As one innovation for reducing the power for a backlight, a method described in Japanese Patent Application Laid-Open Publication No. 11-65531 exists. In the Japanese Patent Application Laid-Open Publication No. 11-65531, a method in which by decompressing image data by an amount of reducing backlight luminance, change of an image is reduced, and therefore, power consumption is reduced is described.
For example, in a histogram of pixel values of an image in which a pixel having luminance of 80% has maximum luminance, by reducing backlight emission to 80%, which is four-fifth of original backlight emission, and multiplying all pixel values of the image to be displayed by a factor of five-fourth accordingly, the same image can be displayed with an amount of light emission of 80%. Furthermore, by using the histogram, attention is focused to pixels in the top several percent. If this portion has luminance of 60%, by suppressing an amount of light emission of the backlight to three-fifth, that is, 60%, and multiplying all pixel values by a factor of five-third accordingly, a similar image can be obtained. In this case, compared with a method using maximum luminance of an image, display with a smaller amount of light emission can be achieved.
As another innovation for reducing the power for a backlight, a method described in Japanese Patent Application Laid-Open Publication No. 3-226716 exists. In the Japanese Patent Application Laid-Open Publication No. 3-226716, a method of controlling a backlight according to external environment is described. For example, external brightness is sensed by an optical sensor and when light-receiving data thereof is lower than a threshold, a backlight is turned off, thereby reducing superfluous power. Furthermore, according to an external-light condition, for example, since reflection of a liquid-crystal panel surface causes poor viewability of a display in outdoor environment with high illuminance, backlight luminance is increased, on the other hand, the backlight luminance is decreased in indoor environment with low illuminance. In this manner, a backlight can be used efficiently by controlling the backlight with a plurality of luminance levels.